Three hundred years ago, it was believed that if all matter were removed from a region of space, a completely empty volume —a vacuum —results. One hundred years ago, it was known that even if all matter were removed from a region of space, that region is not truly empty because it still would contain thermal radiation. At that time, however, it was incorrectly believed that a vacuum could still be created by removing the thermal radiation, such as by cooling the region of space to absolute zero.
More recently, theory has predicted and experimentation has shown that a non-thermal radiation is present everywhere in the universe—even in regions that are otherwise devoid of matter and thermal radiation. This non-thermal radiation is believed to result from random fluctuations occurring at the quantum level that result in a continual creation and destruction of virtual particles. This radiation is often referred to as a “zero point field,” or by the acronym “ZPF,” and the energy that is associated with the field is referred to as “zero point energy,” “vacuum energy,” or simply by the acronym “ZPE.”
In 1948, Hendrik B. J. Casimir theorized that two perfectly conducting, neutral planes that are situated in parallel relation to one another give rise to a mutually attractive force. This force, since referred to as “the Casimir force,” results from the effect that the two planes have on the vacuum energy of a source field, such as an electromagnetic field, between the planes.
The Casimir force is believed to arise solely from the aforedescribed quantum-level activity. The presence of Casimir's two planes, or, in practical applications, two plates, restricts the allowed modes of oscillation of the random fluctuations in the quantum electromagnetic field. In other words, the presence of the plates alters the boundary conditions of the electromagnetic field from free-field conditions. Consequently, the vacuum electromagnetic energy density in the space between the plates is less than the energy density outside of this space (i.e., the number of virtual particles per unit volume in the space between the plates is less than the number of virtual particles per unit volume outside of this space). This difference or gradient in energy density gives rise to a force (i.e., the Casimir force) that pushes the plates together.
While the Casimir force is “real” in the sense that is observable, the quantum electrodynamic (“QED”) theory described above is not the only theory that adequately accounts for its existence. In particular, stochastic electrodynamics (“SED”), which provides a different interpretation, yields the same predictions.
The magnitude of the Casimir force, FC, per unit area is given by the expression:FC(s)=(π2/240)·(h*c/s4)  [1]where: .means “multiply;”                h*=h/(2π)        h is Planck's constant;        c is the speed of light in a vacuum;        s is the separation between the two conducting surfaces.        
It is clear from expression [1] that as s approaches 0, the Casimir force FC(s) becomes large. In fact, the Casimir force per unit area between two plates separated by a distance s of about 0.1 microns is equivalent to the electrostatic force per unit area between the same two plates in the presence of a potential difference of about 100 millivolts.
ZPE is expected to exhibit infinite energy density and to be universally present, and might therefore be a limitless source of energy. This, not surprisingly, has tantalized researchers and spawned several efforts dedicated to ZPE research and commercialization. Such efforts notwithstanding, investigators have not as yet developed devices and methods that are suitable for commercially exploiting ZPE.